The invention disclosed herein pertains to a system for maintaining a predetermined relationship between the operating speeds of two mechanical systems that are driven from a common line shaft which, in turn, is driven rotationally by an electric motor. One example of a use of the invention is to maintain a constant draw or tension in a web that is being drawn by the nip rolls of a chill roll assembly from a web printing press where the web is discharged from the chill roll assembly to a folder. Another example of its use is to maintain a predetermined tension in the web at the infeed end of a printing press between the infeed nip rolls and the printing press.
As is well known, production machines, such as multiple color unit printing presses which process webs, usually require maintaining a predetermined tension or draw, as it is commonly called, in the web in some part of the machine. One example where maintaining a predetermined draw is necessary is in association with a chill roll stand. A paper web after having been printed is pulled from the outfeed printing unit through a hot air dryer which evaporates the volatiles from the ink. The dried ink discharged from the dryer is soft when it is still warm. The paper is drawn by nip rolls in the chill roll stand. The stand has several rotationally driven rolls which are artificially cooled by circulating cold water or refrigerant through them. The web passes over these rolls which cool the ink and cause it to set before the ink reaches the nip rolls which pull or draw the web. If the ink were not hardened before being squeezed by the nip rolls, the ink would be smeared and the printed image would be spoiled.
It is important to maintain proper tension or draw in the web. If the tension in the paper web between the outfeed of the press and the nip rolls of the chill stand is too great the web may stretch and distort the printing or the web may break. A break not only results in scrapping a substantial length of web which was fed out before the press came to a stop but, even as bad or worse, it usually means at least one-half hour of lost production before rethreading the web through the chill roll stand is accomplished.
Before the invention disclosed herein was made systems were provided for controlling web tension. A known system uses a strain wave gearing power transmission device of the type identified by the Trademark "Harmonic Drive", to drive the chill stand. Strain wave gearing devices are described in substantial detail in U.S. Pat. No. 2,906,143 of C. W. Musser which issued on Sept. 29, 1959. In the known tension control system and in the improved system disclosed herein the power input or drive to the strain wave gearing transmission for a chill roll stand is derived from the main or line shaft which also drives all of the color printing units of the press.
The form of strain wave gearing power transmission device of interest here comprises an outer ring gear (circular spline) having internal teeth, a strain gear (flexspline) having external teeth and a strain inducer (strain wave generator). The strain gear resembles a thin metal cylindrical cup which is inside and concentric to the ring gear. A power output shaft extends from the closed end of the strain gear and its external teeth engage with the internal teeth of the ring gear at generally diametrically opposite places. The strain inducer is a cam which is fixed on a shaft and is mounted inside of and coaxially of the flexible toothed wall constituting the strain gear. The strain inducer cam is elliptical. The length of its major axis is such that two opposite sides of the inducer flex the strain gear radially outwardly at two generally diametrically opposite areas to effect engagement of some of the teeth at the two areas on the strain gear with the ring gear teeth. Teeth located in zones between the areas of engagement are not engaged because the minor axis of the strain inducer cam is too short to flex the zones radially outwardly. The strain gear and ring gear have the same diametral pitch but the ring gear teeth have a slightly smaller pitch diameter. The pitch diameter difference results from a number of teeth in the strain gear being fewer than the number of teeth in the ring gear. The difference in the number of teeth is a multiple of the number of areas in which the strain gear is deflected to engage the strain gear with the ring gear. The difference is two teeth when the strain inducer or strain wave generator, as it is otherwise called, constitutes an ellipse having two lobes. Assume for the sake of illustration that the known strain wave gearing device and the device used herein for driving chill rolls rotationally and for maintaining web tension each have an outer ring gear in which there are 202 internal teeth and a strain gear in which there are 200 teeth. The ratio of input to the output is 101 to 100. Assume, for example, that in the chill roll stand drive system which was implemented before the present invention was made, the outer ring gear of the Harmonic Drive is driven rotationally as the power input and the shaft which supports the strain gear (flexspline) is coupled to the chill rolls and is the power output. The strain inducer (wave generator) normally has essentially zero rotational speed for reasons to be discussed later. Assume, for example, that the pulley ratio of the output shaft of the strain gear device is 2:1 relative to the input of the chill roll stand so that to drive the chill rolls at 700 rpm the output shaft speed of the strain gear device should be about 1400 rpm to develop no tension or draw. If the strain inducer shaft is held against rotation, the ratio of the input or driven speed of the ring gear to the output shaft would be 101:100, it would be necessary to drive the ring gear at 1414 rpm to procure nominally proper web tension. However, it is desirable to hold the percent of draw constant even though the speed of the line shaft which delivers power to the printing press units and the chill rolls varies. Thus, a speed control system was adapted to the strain wave gearing through which power is transmitted to the chill roll stand according to the prior art. The control system involves having the output shaft of a servomotor coupled to the strain gear shaft. An encoder in the prior system produces electric pulses at a rate corresponding to the speed of the strain gear output shaft which, in turn, is proportional to the speed of the chill rolls. Another encoder produces pulses at a rate corresponding to the line shaft speed. The pulse signals are compared and otherwise processed. Now, for the sake of clarity that attends use of numerical examples, assume it has been determined that a draw of 0.0033 or 0.33% above zero draw is appropriate. At some moment, for example, the output pulse rate from the chill roll encoder corresponds to a web speed of 602 ft/min. At that moment the line shaft encoder yields output pulses at a rate corresponding to a web speed of about 600 ft/min. In this case, 602/600 equals 1.00333 or 100.333% draw. It is treated as 0.333% draw. So a processor using the pulse count produces an error signal representative of the difference between the actual draw and the desired draw. The error signal is used to energize the servomotor which then drives the strain inducer or wave generator rotationally in an appropriate direction and at a rotational speed to cause the Harmonic Drive to reduce its output. An encoder on the servoshaft produces pulses that are compared with the error signal and when the error signal is nulled by the decline of the strain gear output shaft speed and the chill roll speed equilibrium is reached so that draw supposedly would be restored to 0.33%. Underspeed would be conversely determined.
One problem with the prior art speed control method just outlined results from the strain inducer or wave generator shaft speed rotating at zero rpm for zero percent draw and from raising and lowering the wave inducer speed to get the output shaft of the strain gearing system to track the printing press or other web handling machine line shaft. In other words the prior art reference pulse rate developed by the wave generator shaft encoder is zero at zero draw and the pulse rate is low at the desired percent of draw. Hence the pulse rate incidental to the need for any amount of draw correction is low. A low pulse count from the encoders per unit of time means poor or gross resolution. A consequence is that the accuracy is poor, that is, there is a significant deviation from the desired percentage of draw before the system responds by changing the chill roll speed to get the proper draw.
Another problem with the prior art strain wave gearing power transmission application in a web printing press is that there is excessive bearing stress and heating that results from the driven ring gear shaft running at very high speed relative to the strain inducer or wave generator shaft when the normal speed of the latter is zero as it is in the prior art drive. This results in a dramatic reduction of bearing life. The Harmonic Drive in the prior art chill roll drive and tension control system has a housing in which the outer races of the bearings for a tubular or hollow ring gear shaft are set. The ring gear shaft fits into the inner races of these bearings. The strain inducer shaft is journaled concentrically in the hollow ring gear shaft. Since, in the prior art application of the Harmonic Drive the ring gear shaft, which is the power input, is driven at high speed and the strain inducer shaft is turning slowly or may even be standing still much of the time an output shaft speed mismatch exists. There is a huge difference in the speed of the ring gear shaft relative to the strain inducer shaft so the ball bearings must roll at higher than desired speed. This accounts for the heating and premature wear of the bearings mentioned above. Moreover, sometimes a negative speed direction requires driving the strain inducer in a direction opposite from the ring gear so the speed differential is even greater and so is bearing stress.
In prior art chill roll stand drives there is a toothed pulley on every chill roll shaft and a toothed pulley on the output shaft or the ring gear of the Harmonic Drive so a single drive belt runs on all of the pulleys and drives all of the high inertia chill rolls. In the prior art system the belt must be exceptionally wide which means its tension is high. This puts more load on the journals for the chill rolls which reduces bearing life. The high tension reduces belt life. A wide long toothed belt is much more costly than a plurality of narrower lower tensioned belts which are used according to the invention as will be discussed further later.